On 25 August 2013, Chelsea confirmed that they had agreed a fee of around £30,000,000 to sign Willian, despite the Brazilian had already had a medical at Tottenham. Three days later, it was announced that the deal had been completed. Willian took the number 22 shirt for the 2013-14 season.

On 18 September 2013, Willian made his debut for Chelsea, starting in a 2-1 loss to FC Basel. On 6 October 2013, Willian scored his first goal for the club, coming on as a substitute and netting Chelsea's third goal in a 3-1 win at Norwich.